1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile machine, and a multifunction peripheral including a plurality of functions of those described above.
2. Description of the Related Art
Hitherto, as a full-color image forming apparatus, there is known a so-called tandem configuration in which a plurality of photosensitive drums serving as image bearing members are located alongside an intermediate transfer belt serving as an intermediate transfer member. Further, hitherto, this configuration is also known for such operation that, at a time of monochrome image formation for forming an image only in black, photosensitive drums for the other colors are separated from the intermediate transfer belt, and at a time of forming no image, all the photosensitive drums are separated from the intermediate transfer belt. In addition, Japanese Patent Application Laid-Open No. 2007-212934 proposes a technology for changing, with a simple configuration, the positions of the photosensitive drums among positions at the time of the monochrome image formation, positions at the time when all the photosensitive drums are brought into contact with the intermediate transfer belt to form an image in full color, and positions at the time of forming no image.
By the way, in recent years, for the sake of lower cost, it has been demanded to reduce the number of motors serving as a drive source configured to drive the intermediate transfer belt and so on. Here, it is conceivable to provide a configuration in which a common motor is used to rotate the intermediate transfer belt and to drive a contact-separation mechanism configured to change over a contact-separation state of the photosensitive drums and the intermediate transfer belt among the positions at the time of the monochrome image formation, the positions at the time of the full-color image formation, and the positions at the time of forming no image. In this case, the motor can be driven only in one direction in order to rotate the intermediate transfer belt as well. In other words, when the intermediate transfer belt is rotated in a reverse direction, it is necessary to reversely rotate the photosensitive drums in contact with the intermediate transfer belt at the same time. At this time, there is a fear that toner or the like accumulated on a cleaning blade configured to clean toner or the like in contact with the photosensitive drum may be dragged on the photosensitive drum due to the reverse rotation of the photosensitive drum, with the result that a charging device, a developing device, and so on arranged around the photosensitive drum is stained. Therefore, the motor can be driven only in one direction, and the contact-separation mechanism changes the contact-separation state among the above-mentioned positions in order while the motor is driven in one direction.
In such a configuration configured to change over the contact-separation state of the photosensitive drums and the intermediate transfer belt among the respective positions in order, when the contact-separation state is changed over at a time of performing or completing the monochrome image formation, the photosensitive drums are once changed over to the positions at the time of the full-color image formation. In general, at the time of the monochrome image formation, the driving of the color photosensitive drums other than the one for black is stopped. In contrast, the intermediate transfer belt is in a rotating state because the motor is being driven in order to change over the contact-separation mechanism. Accordingly, when the photosensitive drums are once changed over to the positions at the time of the full-color image formation, the stopped color photosensitive drums and the rotating intermediate transfer belt rub each other. Such rubbing may cause a flaw in the photosensitive drums and the intermediate transfer belt, thereby causing an image failure such as a streaked image.